Direct analysis of samples using atmospheric pressure ionization (API) provides a more rapid method for analysis of volatile and semi-volatile compounds than vacuum solids probe methods and can be accomplished on commercial API mass spectrometers. With only a simple modification to either an electrospray (ESI) or atmospheric pressure chemical ionization (APCI) source, solid as well as liquid samples can be analysed quickly. The method acts as a fast solids/liquid probe introduction as well as an alternative to relatively direct analysis in real time (available as DART® technology from JEOL, Tokyo, Japan) and desorption electrospray ionization (DESI) methods for many compound types. Vaporization of materials occurs in a hot nitrogen gas stream flowing from an ESI or APCI probe. Ionization of thermally induced vapors occurs by corona discharge under standard APCI conditions. Accurate mass and mass-selected fragmentation are achieved as is the ability to obtain ions from biological tissue, paper currency, and other objects placed in the path of the hot nitrogen stream.
The introduction of direct analysis in real time and desorption electrospray ionization (DESI) methods has led to the development of a technique known as atmospheric-pressure solids analysis probe (available as ASAP™ technology from M&M Mass Spec Consulting, Hockessin, Del.) Atmospheric-pressure solids analysis probe is a method for rapidly analyzing volatile or semi-volatile liquid or solid materials using only slightly modified commercial electrospray (ESI) or atmospheric pressure chemical ionization (APCI) ion sources. The technique is similar to the mass spectrometric methods used for the analysis of drugs and explosives at atmospheric pressure and overlaps significantly with the direct analysis in real time and DESI methods.
Atmospheric-pressure solids analysis probe can be used in conjunction with time-of-flight (TOF), Orbitrap, double-focusing magnetic sector, and Fourier transform ion cyclotron resonance mass spectrometers to obtain high mass resolution and accurate mass measurement. Fragmentation is minimal in APCI relative to electron ionization; thus, mixtures are more easily analyzed than with solids probe electron ionization. However, fragmentation can be enhanced by so-called cone-voltage fragmentation or by collision-induced fragmentation in conjunction with MS/MS of selected precursor ions. Because vaporization and ionization with atmospheric-pressure solids analysis probe occurs at atmospheric pressure, a mass spectrum can be acquired quickly from solid and liquid materials such as pure compounds, mixtures, biological tissue, and plastics. The attributes of speed of analysis, accurate mass measurement, and fragmentation of selected precursor ions is achieved as is the sensitivity of atmospheric-pressure solids analysis probe, demonstrated by detecting drugs present on paper currency.
Atmospheric-pressure solids analysis probe can be implemented on any ESI/APCI ion source by simple introduction of the material of interest into a stream of heated gas near the ionization region. The heated gas used in experiments has been obtained from either the heated nitrogen gas used for desolvation in an ESI or an APCI probe. The atmospheric-pressure solids analysis probe method uses the sources of heated gas available with ESI/APCI ion sources for volatilization; otherwise it is similar to the techniques for analysis of drugs removed from paper currency and for analysis of explosives wiped from solid surfaces. The material used to introduce samples into the hot gas stream needs to be free of volatile components. Melting point tubes are inexpensive and ideal for this application. Ionization is by a corona discharge at atmospheric pressure using the standard voltages and discharge apparatus supplied with the APCI ion source (alternatively, ionization can be by photoionization). When introducing samples into the ionization region, it is preferred that the ion source be enclosed and vented to a hood to prevent vapors from reaching the laboratory air.
A known atmospheric pressure solids analysis probe comprises a one-piece probe capable of holding a disposable or easily cleaned sample holding device which can be partially inserted into an atmospheric pressure ionization source comprising an enclosure or chamber, thus allowing the sample on the holding device to be inserted into the atmospheric pressure ionization region of a mass spectrometer.